Marketing of containers made of synthetic resin, particularly of PET (Polyethylene Terephthalate), has been widely developed mainly regarding large sized containers for soft drinks. Recently, usage of a heat resistant container has been required and, hence, much study and development has been carried out.
Synthetic resin containers of the heat-resistant multilayer type are generally composed of a mouth portion with which a lid is screw engaged, and a shell portion continuous to the lower end of the mouth portion. Such containers are manufactured by forming a parison formed of a main resin and a heat-resistant resin and forming a shell portion by holding a mouth portion of the parison and by performing a drawing blow process.
The formation of the heat resistant container involves such problems as degradation of the heat resistant deformability, chemical resistance and strength of the mouth portion because the mouth portion is not drawn and maintained as it is injected. In order to obviate these problems, conventional art provides such methods as (a) a method in which the mouth portion is heat crystallized, (b) a method in which the mouth portion is formed-by two-color formation of the heat-resistant resin and (c) a method in which the mouth portion is formed by preliminarily forming an outer periphery of the mouth portion with the heat-resistant resin and carrying out insertion formation in the preliminarily formed product.
However, the mouth portion formed by the method (a) involves faults such that diameters of a screw thread and a screw root thread of the mouth portion as well as the size thereof are not stably formed because of shape deformation during the heat crystallization process of the mouth portion, the sealing performance thereof is degraded because of the deformation of a top surface of a seal portion and this method is not applicable in a case where a totally transparent container is required because of the opacity due to the crystallization. Furthermore, a crystallizing process is additionally needed, resulting in less productivity of products and an increased cost.
The mouth portion formed by the method (b) has an insufficient layer-to-layer adhesive strength between the main resin and the heat-resistant resin and, in addition, requires a plurality of molding devices for the manufacturing thereof, involving complicated processes and, hence, resulting in cost increasing.
The mouth portion formed by the method (c) has also insufficient layer-to-layer adhesive strength between the main resin and the heat-resistant resin and also requires a plurality of molding devices. An insert device is further needed, thus also involving complicated processes and cost increasing.
There has been proposed a further method for eliminating the defects described above in which the main and heat resistant resins are co-injected from the lower portion to thereby form a parison and the parison is thereafter subjected to drawing blow formation to form a heat-resistant multilayer container. In such a parison formation method, it is required to utilize a hot runner for uniformly co-injecting the molten resin of the main and heat-resistant resins in accordance with the predetermined timing and injection quantity. The applicant of the present invention has developed a co-injection molding machine provided with a hot runner in which a hot runner portion, except a hot-runner branching point and a portion near that point, is composed of a pair of molten resin flow passages extending mutually closely and having the same cross sections and there is provided an area which combines two resin flows to the branching point (Japanese Patent Laid-open Publication No. 61-252977).
It is required to discriminate the performance and the quality of the heat resistant containers manufactured by the methods described above. The discrimination for the good performance and quality is based on a standard such as that the heat-resistant resin is concentrated to the mouth portion of the heat-resistant multilayer container.
The conventional discrimination includes a visual checking method, but this method is not suitable for discrimination in the case of the main and heat resistant resins both having the same color or both being transparent. It may be possible to discriminate the performance and the quality by a non-destructive technique based on a sampling method, but this method lacks reliability of discrimination because this method involves unstable factors in the manufacturing process and, hence, fears of causing unexpected faults when compared with single-layered containers.